The nervous system is your body’s communication network, responsible for everything from conscious thought to the automatic beating of your heart. It consists of three main structures: the brain, the spinal cord, and a branching web of nerves that reaches every corner of your body. Together, these structures collect information from your environment, process it, and send instructions back out to your muscles, organs, and glands.
Two Main Divisions
The nervous system splits into two major parts. The central nervous system (CNS) is the command center, made up of your brain and spinal cord. Your brain reads incoming signals and coordinates how you think, move, and feel. The spinal cord serves as the main highway connecting the brain to the rest of the body.
The peripheral nervous system (PNS) is everything outside that command center. It’s a vast network of nerves that branch out from your spinal cord and extend into your limbs, torso, and face. These peripheral nerves carry sensory information up to the brain and motor commands back down to your muscles. Some are one-way streets: sensory nerves only send information toward the brain, while motor nerves only carry instructions away from it.
The Cells That Make It Work
The basic unit of the nervous system is the neuron, a specialized cell built for rapid communication. The human brain alone contains roughly 95 to 100 billion of them. Each neuron has three key parts. Dendrites are the branching, tree-like extensions that receive incoming signals from other cells. The cell body (or soma) houses the nucleus and the machinery that keeps the cell alive, handling tasks like building proteins and generating energy. The axon is a long, cable-like projection that carries electrical signals away from the cell body toward the next neuron or target cell.
Neurons don’t work alone. They’re supported by glial cells, which outnumber neurons in many parts of the nervous system. One type produces the insulating coating (myelin) that wraps around axons and dramatically speeds up signal transmission. Another type acts as the brain’s cleanup crew, clearing away damaging waste products and maintaining the chemical balance neurons need to function. A third type serves as the brain’s immune cells, pruning unnecessary connections during development and responding to injury or disease.
How Signals Travel
Neurons communicate through a two-step process that converts electrical signals into chemical ones and back again. At rest, the inside of a neuron is slightly negatively charged compared to the outside, sitting at about -70 millivolts. When enough stimulation reaches the neuron, charged particles (ions) rush across the cell membrane and briefly flip that voltage. This rapid electrical spike, called an action potential, lasts about one millisecond and races down the length of the axon.
The speed of that signal varies enormously depending on the nerve fiber. Small, uninsulated fibers carrying pain information transmit at roughly 0.5 to 2 meters per second, about walking pace. Large, insulated fibers handling touch and body position awareness can hit 80 to 120 meters per second, or up to 268 miles per hour.
When the electrical signal reaches the end of an axon, it triggers the release of chemical messengers called neurotransmitters. These molecules cross a tiny gap (only 20 to 40 nanometers wide) between the sending neuron and the receiving cell. Once they land on receptors on the other side, they open channels that let ions flow in, converting the chemical signal back into an electrical one. This chain reaction repeats billions of times every second across your nervous system.
Voluntary Movement and Senses
The somatic nervous system handles everything you consciously control and consciously sense. It carries signals from your skin, ears, nose, and tongue to your brain, giving you the experience of touch, hearing, smell, and taste. (Sight is the one exception; your retina and optic nerve connect directly to the brain without routing through the somatic system.) It also carries motor commands from your brain to your skeletal muscles, allowing you to walk, type, speak, and pick things up.
This system is what lets you feel a tap on your shoulder and turn your head in response. Sensory nerves below the neck relay information to the spinal cord first, which then passes it up to the brain. Motor commands travel the reverse path, from brain to spinal cord to muscle.
The Autonomic System: Automatic Control
Many critical functions happen without any conscious effort on your part. Your heart beats, your lungs expand, your stomach digests food. The autonomic nervous system manages all of this, and it has two opposing branches that work like a gas pedal and a brake.
The sympathetic branch is your body’s accelerator, activating the fight-or-flight response. When you perceive a threat, it increases your heart rate, speeds up your breathing, and redirects blood flow away from your digestive system toward your skeletal muscles. Sweat production ramps up. Your body is preparing to act.
The parasympathetic branch does the opposite. Often called the “rest and digest” system, it slows the heart rate, promotes digestion, and generally helps the body conserve energy and recover. These two branches constantly balance each other, adjusting your internal state moment to moment throughout the day.
Your Gut Has Its Own Nervous System
There’s a third division that often surprises people. The enteric nervous system is a self-contained network of roughly 200 million neurons embedded in the walls of your digestive tract. That’s more neurons than in your spinal cord, and it’s often called the “second brain” for good reason: it can regulate digestion almost entirely on its own, without input from the brain.
This independence is why digestion largely happens outside your conscious awareness. The enteric nervous system coordinates the muscular contractions that move food through your gut, manages the release of digestive enzymes, and monitors conditions inside the intestines. It does communicate with the brain through what’s known as the gut-brain axis, a two-way signaling pathway that helps integrate digestive function with the rest of the body. But the day-to-day work of breaking down food and absorbing nutrients is handled locally, freeing the brain to focus on other tasks.
The Nervous System Can Rewire Itself
For decades, scientists believed the brain’s wiring was essentially fixed after childhood. That turned out to be wrong. The nervous system retains a remarkable ability to reorganize itself throughout life, a property called neuroplasticity. It forms new connections between neurons, strengthens frequently used pathways, and can even reassign functions from damaged areas to healthy ones.
This flexibility is what allows you to learn a new language at 50, recover some function after a stroke, or adapt to the loss of a sense by sharpening others. Neuroplasticity supports learning and memory at every age. It also has a downside: the same rewiring ability can reinforce harmful patterns, such as chronic pain pathways that persist after an injury has healed. The process works in both directions, building beneficial connections and, sometimes, maladaptive ones.